The present invention relates to an alloy type thermal fuse, more particularly to improvement in an alloy type thermal fuse of an operating temperature of 65 to 75° C., and also to a fuse element which constitutes such a fuse, and which is made of a low-melting fusible alloy.
In a conventional alloy type thermal fuse, a low-melting fusible alloy piece to which a flux is applied is used as a fuse element. When an electric apparatus on which such a fuse is mounted abnormally generates heat, therefore, a phenomenon occurs in which the low-melting fusible alloy piece is liquefied by the generated heat, the molten metal is spheroidized by the surface tension under the coexistence with the flux that has already melted, and the alloy piece is finally broken as a result of advancement of the spheroidization, whereby the power supply to the apparatus is interrupted.
The first requirement which is imposed on such a low-melting fusible alloy is that the solid-liquid coexisting region between the solidus and liquidus lines is narrow.
In an alloy, usually, a solid-liquid coexisting region exists between the solidus and liquidus lines. In this region, solid-phase particles are dispersed in a liquid phase, so that the region has also the property similar to that of a liquid phase, and therefore the above-mentioned breakage due to spheroidization may occur. As a result, there is the possibility that a low-melting fusible alloy piece is spheroidized and broken in a temperature range (indicated by ΔT) which is lower than the liquidus temperature (indicated by T), and which belongs to the solid-liquid coexisting region. Therefore, a thermal fuse in which such a low-melting fusible alloy piece is used must be handled as a fuse which operates at a fuse element temperature in a range of (T-ΔT) to T. As ΔT is smaller, or as the solid-liquid coexisting region is narrower, the operating temperature of a thermal fuse is less dispersed, so that a thermal fuse can operate at a predetermined temperature in a correspondingly strict manner. Therefore, an alloy which is to be used as a fuse element of a thermal fuse is requested to have a narrow solid-liquid coexisting region.
The second requirement which is imposed on such a low-melting fusible alloy is that the electrical resistance is low. When the temperature rise by normal heat generation due to the resistance of the low-melting fusible alloy piece is indicated by ΔT′, the operating temperature is substantially lower by ΔT′ than that in the case where such a temperature rise does not occur. Namely, as ΔT′ is larger, the operation error is substantially larger. Therefore, an alloy which is to be used as a fuse element of a thermal fuse is requested to have a low specific resistance.
Conventionally, as a fuse element of an alloy type thermal fuse of an operating temperature of 65 to 75° C., known is a Bi—Pb—Sn—Cd alloy (50% Bi, 26.7% Pb, 13.3% Sn, and 10% Cd (% means a weight percent (the same is applicable in the following description))) which is eutectic at 70° C. However, the alloy is not suitable to environment conservation which is a recent global request, because, among Pb, Cd, Hg, and Tl which are seemed to be harmful to the ecological system, Pb and Cd are contained in the alloy.
In order that the size of an alloy type thermal fuse is reduced in accordance with the recent tendency that electric or electronic apparatuses are further miniaturized, a fuse element must be made very thin (about 300 μm). However, the alloy which contains a large amount of Bi is so fragile that a process of drawing the alloy into such a very thin wire is hardly performed. In such a very thin fuse element, moreover, the relatively high specific resistance of the alloy composition cooperates with the thinness to extremely raise the resistance, with the result that an operation failure due to self-heating of the fuse element inevitably occurs.
Also an In—Bi alloy (66.3% In, and 33.7% Bi) which is eutectic at 72° C. is known. In the alloy, a solid phase transformation occurs at a temperature between 53° C. and 56° C. Because of relative relationships between the temperature and the operating temperature of 65 to 75° C., the temperature coincides with a temperature to which a fuse element is exposed during a normal operation of an apparatus. Therefore, strain due to a solid phase transformation is produced in the fuse element. As a result, the resistance of the fuse element is raised, and there arises the possibility that an operation failure due to self-heating of the fuse element occurs.
To comply with this, the inventor has proposed that an alloy composition of 25 to 35% Bi, 2.5 to 10% Sn, and the balance In is used as a fuse element of an alloy type thermal fuse in which the operating temperature is in the range of 65 to 75° C., no toxic metal is contained, the diameter of the fuse element can be reduced to about 300 μmφ, and self-heating can be suppressed to enable the fuse element to normally operate (Japanese Patent Application Laying-Open No. 2001-291459).
In the alloy type thermal fuse, because of In and Bi of the above compound ratios, the melting point is provisionally set to the vicinity of 70° C. and adequate ductility required for drawing into a thin wire is obtained, and, because of the blending of Sn, the range of the solidus and liquidus temperatures is finally set to 65 to 75° C. and the specific resistance is set to be low. When the lower limit of the compound ratio of Sn is smaller than 2.5%, the amount of Sn is so insufficient that the above-mentioned solid phase transformation cannot be effectively prevented from occurring. When the upper limit of the compound ratio of Sn is larger than 10%, an In—Bi—Sn eutectic structure (58% In, 29% Bi, and 13% Sn) of a melting point of 62° C. appears, and the range of the solidus and liquidus temperatures cannot be set to be between 65° C. and 75° C. In this composition, since the total amount of In and Sn which have a relatively lower specific resistance is larger than the amount of Bi of a higher specific resistance, the whole specific resistance can be sufficiently lowered. Even in the case of a very thin wire of 300 μmφ, a low resistance of a fuse element can be easily attained (25 to 35 μΩ·cm), a solid phase transformation does not occur in a lower temperature side of an operating temperature of 65 to 75° C., and also a resistance change due to a solid phase transformation of a fuse element at a temperature during a normal operation of an apparatus with respect to the operating temperature of 65 to 75° C. can be eliminated. Therefore, the operating temperature of the thermal fuse can be set to be within a range of ±5° C. with respect to 70° C.
In the alloy composition of the fuse element, In is 72.5 to 55% or occupies the majority of the composition. Since In is expensive, the production cost of such a fuse element is inevitably increased.
Such a thermal fuse is repeatedly heated and cooled by heat cycles of an apparatus. During the heat cycles, therefore, thermal stress of α·Δt·E where α is the coefficient of thermal expansion of the fuse element, Δt is the temperature rise, and E is the Young's modulus is generated within the elastic limit, and compression strain of α·Δt is imposed. In the above-mentioned alloy composition (25 to 35% Bi, 2.5 to 10% Sn, and the balance In), because of the large content of In (55 to 72.5%), the elastic limit is so small that a large slip is caused in the interface between different phases in the alloy structure by strain which is smaller than compression strain of α·Δt. When the strain is repeated, the sectional area and the length of the fuse element are changed, and the resistance of the fuse element itself becomes unstable. In other words, the thermal stability cannot be guaranteed.